1. Field of the Invention
The present invention relates to a display device in which a light-emitting element is provided in each of the pixels. In particular, the invention relates to a display device of the active matrix type in which a transistor is provided for each of the pixels to control the emission of light from the light-emitting elements. The invention further relates to electronic devices using the display device.
2. Description of the Related Art
There has been proposed a display device of the active matrix type in which a light-emitting element and a transistor are arranged for each of the pixels to control the emission of light from the light-emitting element. In particular, attention has been given to the display device of the active matrix type using thin-film transistors (hereinafter referred to as TFTs) as transistors.
The light-emitting element has a first electrode and a second electrode, and changes its brightness depending upon the amount of current that flows across the first electrode and the second electrode. As the light-emitting element, attention has been given to an element (referred to as EL element) which utilizes electroluminescence. Attention has been given particularly to a display device (hereinafter referred to as EL display device) using the EL elements (EL element utilizing an organic substance is also referred to as an organic EL element or an OLED (organic light-emitting diode) element (OLE Device, OELD).
Here, the EL element stands for the one having an anode, a cathode and an EL layer held between the anode and the cathode. The anode and the cathode correspond to a first electrode and to a second electrode. Upon applying a voltage across these electrodes, a current flows between the electrodes. The EL element emits light depending upon the amount of electric current that flows.
The EL layer can be constructed in a stacked layer structure. A representative example may be a stacked layer structure “positive hole transporting layer/light-emitting layer/electron transporting layer” proposed by Tang et al. of Kodak Eastman Co. Here, the electron transporting layer is the one made of a material (hereinafter also referred to as electron transporting material) exhibiting a higher electron mobility (electron transporting function) than a positive hole mobility. The light-emitting layer is the one made of a material having light-emitting property (light-emitting function)(hereinafter referred to as light-emitting material). The positive hole transporting layer is the one made of a material (hereinafter referred to as positive hole transporting material) exhibiting a higher positive hole mobility (positive hole transporting function) than an electron mobility. There can be further employed a structure in which positive hole injection layer/positive hole transporting layer/light-emitting layer/electron transporting layer, or positive hole injection layer/positive hole transporting layer/light-emitting layer/electron transporting layer/electron injection layer, are stacked in this order on the anode. The light-emitting layer may be doped with a fluorescent coloring matter. Here, the electron injection layer is the one made of a material (hereinafter referred to as electron injection material) having electron injection property (electron injection function) for receiving electrons from the cathode. Further, the positive hole injection layer is the one made of a material (hereinafter referred to as positive hole injection material) having positive hole injection property (positive hole injection function) for receiving positive holes from the anode. The layers formed between the cathode and the anode are all referred to generally as EL layer. When a predetermined voltage is applied from the pair of electrodes (anode and cathode) to the EL layer having the above structure, carriers undergo the recombination in the EL layer to emit light. The layers held between the anode and the cathode of the EL element are generally described as the EL layer.
The EL display device has such advantages as excellent response characteristics, operates at a low voltage and offers a wide viewing angle, and is drawing attention as a flat panel display of the next generation. In the EL display device of the active matrix type, the method of controlling the brightness of EL elements in-the pixels by flowing a predetermined current between the anode and the cathode of the EL elements, is called current-controlled type.
Described below is a constitution of the pixel of the current-controlled type. Namely, described below is a pixel of the current-controlled type in which the signal lines (source signal lines) of pixels are served with a current signal (hereinafter referred to as signal current) that linearly corresponds to brightness data expressed by a video signal.
Each pixel has a TFT which receives a signal current as a drain current and a capacitor unit for holding a gate voltage of the TFT. Namely, the pixel has a function for converting the input signal current into a voltage (gate voltage) and for holding the voltage. Further, each pixel has a function for converting the voltage stored in the capacitor unit into a current again, and continues to flow the converted current into the EL element even after the signal current is no longer input from the source signal line. The current flowing into the EL element changes upon changing the signal current input to the source signal line, whereby the brightness of the EL element is controlled to express the gray scale.
FIG. 10 illustrates a conventional pixel of the current-controlled type, and its driving method (see, for example, patent literature 1). In FIG. 10, a pixel is constituted by an EL element 709, a select TFT 704, a drive TFT 707, a current TFT 706, a capacitor element (holding capacitor) 708, a holding TFT 705, a source signal line S, a first gate signal line G, a second gate signal line GH and a power line W (Patent literature 1: JP-A-2001-147659).
Either the source terminal or the drain terminal of the TFT is called first terminal, and the other one is called second terminal.
The gate electrode of the select TFT 704 is connected to the first gate signal line G. A first terminal of the select TFT 704 is connected to the source signal line S, and a second terminal is connected to a first terminal of the current TFT 706 and to a first terminal of a holding TFT 705. A second terminal of the current TFT 706 is connected to the power source line W. A second terminal of the holding TFT 705 is connected to one electrode of the holding capacitor 708 and to the gate electrode of the drive TFT 707. The holding capacitor 708 is connected to the power line W on the side that is not connected to the holding TFT 705. The gate electrode of the holding TFT 705 is connected to a second gate signal line GH. The first terminal of the drive TFT 707 is connected to one electrode 709a of the EL element 709, and the second terminal thereof is connected to the power line W. Another electrode 709b of the EL element 709 is maintained at a predetermined potential. The value of the signal current input to the source signal line S is controlled by a video signal input current source 777. The electrode 709a of the EL element 709 is called pixel electrode and another electrode 709b is called opposing electrode.
Here, the drive TFT 707 and the current TFT 706 have the same polarity, and it is considered that the Id-Vgs characteristics of the drive TFT 707 are equivalent to the Id-Vgs characteristics of the current TFT 706. There is further illustrated a pixel of a constitution in which the select TFT 704 and the holding TFT 705 are the N-channel TFTs, the drive TFt 707 and the current TFT 706 are the P-channel TFTs, and the pixel electrode 709a is the anode.
How to drive the pixel of the constitution of FIG. 10 will now be described with reference to FIGS. 11A–C and 12. In FIGS. 11A–C, the select TFT 704 and the holding TFT 705 are expressed as switches so that their on/off state can be easily understood. The pixel states (TA1) to (TA3) are corresponding to the states of periods TA1 to TA3 in the timing chart of FIG. 12.
In FIG. 12, G-1 and G-2 denote potentials of the first gate signal line G and of the second gate signal line GH. Further, |Vgs| is an absolute value of the gate voltage (gate-source voltage) of the drive TFT 707. IEL denotes a current flowing through the EL element 709, and Ivideo denotes a current determined by the video signal input current source 777.
In the period TA1, the select TFT 704 and the holding TFT 705 are turned on due to signals of the first gate signal line G and of the second gate signal line GH. Thus, the power line W is connected to the source signal line S through the current TFT 706, holding TFT 705 and select TFT 704. The current Ivideo determined by the video signal input current source 777 flows into the source signal line S. When a steady state is assumed after the passage of a sufficient period of time, therefore, the drain current of the current TFT 706 becomes Ivideo. Thus, the gate voltage corresponding to the drain current Ivideo of the current TFT 706 is held by the holding capacitor 708. Then, in the period TA2, a signal on the second gate signal line GH changes and the holding TFT 705 is turned off. The drain current Ivideo flows into the drive TFT 707. Thus, the signal current Ivideo flows into the EL element 709 from the power line W through the source and drain of the drive TFT 707. The EL element 709 emits light maintaining a brightness corresponding to the signal current Ivideo.
In the constitution shown in FIG. 10, a current flows from the anode 709a of the EL element 709 to the cathode 709b owing to the above method. When the EL element 709 emits light, the second terminal of the current TFT 706 corresponds to the source terminal, and the first terminal corresponds to the drain terminal. Further, the second terminal of the drive TFT 707 corresponds to the source terminal, and the first terminal corresponds to the drain terminal.
In the next period TA3, the signal on the first gate signal line G changes, and the select TFT 704 is turned off. Even after the select TFT 704 is turned off, the signal current Ivideo continues to flow into the EL element 709 from the power line W through the source and drain of the drive TFT 707, and the EL element 709 continues to emit light.
A series of operations of periods TA1 to TA3 are called signal current Ivideo writing operation. In this case, the signal current Ivideo is varied in an analog manner to change the brightness of the EL element 709 and to express the gray scale.
In the above current-controlled display device, the drive TFT 707 operates in the saturated region. Here, the drain current of the drive TFT 707 is determined by the signal current input from the source signal line S. That is, if the current characteristics are the same between the drive TFT 707 and the current TFT 706 in the same pixel, the drive TFT 707 automatically varies its gate voltage to continuously flow a constant drain current irrespective of dispersion in the threshold voltage and in the mobility.
In the EL element, a relationship (I-V characteristics) between the electric current that flows and the voltage across the anode and the cathode, varies depending upon the environmental temperature at which the EL element is used and upon the deterioration of the EL element. However, the pixel of the current-controlled type is capable of maintaining the current flowing into the EL element nearly constant irrespective of the environmental temperature at which the EL element is used and the deterioration of the EL element.
FIG. 9 illustrates a change in the deterioration of the EL element with the passage of time of when the current flowing into the EL element is maintained constant, and wherein the ordinate represents the brightness L of the EL element and the abscissa represents the time t. A curve 900 represents a change in the brightness of when the current flowing into the EL element is maintained constant. When the time is t0, the brightness L of the EL element is supposed to be 100%. The EL element undergoes the deterioration depending upon the time in which a predetermined current continues to flow. Accordingly, the brightness decreases even while the same current is flowing between the anode and the cathode of the EL element.
In the current-controlled type pixel, therefore, a problem arises concerning the dispersion in the brightness due to deterioration of the EL element.